1. Field of the Invention
The present invention relates to a light scanning apparatus having a plurality of light sources, an image forming apparatus, and a light power control method.
2. Description of the Related Art
Conventionally, image forming apparatuses and image reading apparatuses using a plurality of light-emitting elements (light sources) have been proposed. These apparatuses execute control to maintain a predetermined amount of a light beam output from each light-emitting element. Japanese Patent Laid-Open No. 63-292763 describes an image reading apparatus which causes a plurality of LEDs to emit light sequentially and determines the light power correction value of each element to maintain a predetermined light power, thereby controlling the light power of each LED.
Japanese Patent Laid-Open No. 63-142392 describes a method which applies one of three correction data stored in a ROM in advance to a plurality of LEDs used in a printer, thereby making the light powers of the LEDs uniform.
Assume that a surface emitting laser (a surface emitting laser element which extracts a laser beam in a direction perpendicular to the active layer; also referred to as a “VCSEL”) is applied as the light source of an image forming apparatus. Only one of a plurality of light-emitting points provided on the surface emitting laser is selected at random and caused to emit light to acquire data necessary for control. The light powers of the remaining light-emitting points are also controlled on the basis of this data. This method can shorten the control time necessary for light power control because light emission, data acquisition, and light power control need not be performed for each light-emitting point.
In this control method, however, the accuracy of light power control varies between the remaining beams because the light power of only one reference beam is monitored. Especially, the current-light power characteristic always varies between the light-emitting points of the surface emitting laser. Hence, the control accuracy may also vary.
FIG. 12 is a graph showing an example of current-light power characteristics in a surface emitting laser. Reference numeral 1201 denotes a characteristic of a first light-emitting point; and 1202, a characteristic of a second light-emitting point. As is apparent from FIG. 12, when the current decreases by ΔI, the light power of the reference beam decreases by ΔP. When the current decreases by ΔI at the second light-emitting point, the light power decreases by ΔP′ that is larger than ΔP. This indicates that the variation of light power changes between the light-emitting points even when the variation of current does not change. In light power control, when the light beam output from the first light-emitting point is used as the reference beam, the accuracy of light power control at the second light-emitting point relatively decreases.
Especially, the characteristic of a red surface emitting laser varies to a large extent between the light-emitting points as compared to the characteristic of an infrared surface emitting laser because of the manufacture processes, materials and physical factors. That is, in the red surface emitting laser, the accuracy of light power control more readily decreases. However, to further raise the resolution of an electrophotographic type image forming apparatus, a red surface emitting laser becomes necessary in certain situations because it can ensure a smaller spot size than an infrared surface emitting laser. It will be impossible avoid the above-described problem of light power control in the future.